1. Technical Field
The present invention relates to a copper foil for a printed wiring board, and particularly, to a copper foil for a flexible printed wiring board.
2. Related Art
Printed wiring boards have been significantly developed for the last half century, and these days, the printed wiring boards escalate in almost all of electronics. High-density packaging of components to be mounted thereon and raising a signal frequency have been progressed together with increasing needs to downsize and offer high performance to electronics in recent years, and printed wiring boards are required to have miniaturization in conductive patterns (fine pitch), high frequency solutions, and the like.
Generally, printed wiring boards are manufactured through the steps of: adhering an insulating substrate to a copper foil to form a copper-clad laminate; and forming a conductive pattern on the copper foil surface by etching. Therefore, copper foils for a printed wiring board are required to have adhesiveness to an insulating substrate and etching performance.
A surface treatment for forming irregularities on the surface of a copper foil, called a roughening treatment, is generally performed as a technique for improving the adhesiveness to an insulating substrate. For example, there is a method in which fine irregularities are formed by electrodepositing many copper grains to an M surface (roughened surface) of an electrolytic copper foil in a dendritic or spherule form using an acid copper sulfate plating bath to enhance the adhesiveness by an anchor effect. After the roughening treatment, a chromate treatment, a treatment using a silane coupling agent, or the like is generally performed in order to further improve the adhesive characteristics.
A method of forming a metal layer such as tin, chromium, copper, iron, cobalt, zinc, and nickel or an alloy layer thereof on the surface of a copper foil has also been known.
However, the method of improving the adhesiveness by the roughening treatment is disadvantageous in the formation of fine lines. That is, when an interval between conductors is reduced due to fine pitch, the roughened portion remains on the insulating substrate after formation of a circuit by etching and may cause insulation deterioration. When etching is performed over the entire roughened surface in order to prevent the above problem, a long etching time is required and a predetermined wire width may not be maintained.
A method of providing, for example, a Ni layer or a Ni—Cr alloy layer on the surface of a copper foil has a lot of room for enhancement in basic characteristics, i.e., adhesiveness to an insulating substrate. In the case of a method of providing, for example, a Cr layer on the surface of a copper foil, relatively high adhesiveness is obtained, but etching performance is poor, and thus there is a problem in that “etching residues” which are Cr remaining on the insulating substrate surface are easily generated after performing an etching treatment for forming a conductive pattern.
Accordingly, in recent years, a technique for simultaneously obtaining superior adhesiveness to an insulating substrate and superior etching performance by forming a first metal layer on the surface of a copper foil and by forming a thin Cr layer as a second metal layer having superior adhesiveness to an insulating substrate on the first metal layer so that superior etching performance is obtained has been studied and developed.
As such a technique, for example, Patent document 1 (JP 2006-222185 A) discloses that a surface-treated copper foil for a polyimide flexible copper-clad laminate, which has a high peeling strength with respect to a polyimide resin layer and is excellent in insulating reliability, etching characteristics during the formation of a wiring pattern, and flexural properties, is obtained by providing a Ni alloy layer containing Ni in an amount of 0.03 mg/dm2 to 3.0 mg/dm2 and/or a Cr layer and/or a Cr alloy layer as a surface-treated layer containing Cr in an amount of 0.03 mg/dm2 to 1.0 mg/dm2 on a Ni layer.